Chromium has long had a presence in industrial coatings. The chemical and mechanical properties of chromium render it suitable for a number of applications including engineering applications and decorative applications. Engineering applications are generally defined as applications where the chromium layer is relatively thick (for example greater than 10 μm) whereas decorative applications normally have a thin layer of around 0.2-1.0 μm. In decorative applications the chromium deposit typically exhibits a specular metallic finish with a slight bluish tint.
The current invention, in one embodiment, is directed primarily to the application field of decorative coatings. The properties of chromium that make it suitable for these decorative applications include its attractive color and high hardness, which even with thin coatings provides for some scratch resistance.
The most cost-effective method of depositing substantial layers of chromium is electrodeposition which is traditionally used to deposit chromium from electrolytes containing hexavalent chromium compounds. Such electroplating baths have a poor efficiency and, as such, the building up of thick chromium coatings is not cost effective. Therefore, to provide resistance to the elements and corrosion protection for the base substrate one typical practice first applies a thick coating of nickel (normally between 10 and 50 μm) and then applies only a thin layer of chromium over the top of this nickel coating. The nickel coating may consist of a single layer or a combination of two, three or even four distinct layers to provide for maximum corrosion protection of the substrate material and to maintain the decorative appearance of the coating. Depending on the substrate material of the article, other pretreatment and metallic coatings layers may be applied prior to the nickel undercoat, for example in the case of parts manufactured from ABS or other non-conductive materials, or from zinc diecast materials. Such treatments are generally well known to those skilled in the art.
Typical commercial applications for these types of decorative coatings include shop fittings, sanitary fittings (such as taps, faucets and shower fixings) and automobile trim (such as bumpers, door handles, grilles and other decorative trim), by way of example and not limitation.
Traditionally the corrosion resistance of the aforementioned nickel/chromium deposits has been measured by a method known as the CASS test, applied according to the internationally recognized standard ASTM B368. This consists of exposing the electroplated articles to a corrosive fog spray (comprising aqueous sodium chloride, copper chloride and acetic acid) in an enclosed chamber at a temperature of 49° C. After a set exposure time the appearance of the articles is examined and the degree of their corrosion protection is assessed according to ASTM B537.
The degree of corrosion protection required depends upon the likely environment to be encountered by the electroplated article (for example exterior or interior automotive trim). The typical thicknesses and types of deposits recommended are summarized in the ASTM standards B456 and B604. Typically automotive companies will require parts for interior trim to be able to withstand 24 hours exposure to CASS, whereas exterior parts will typically require protection against exposure times of up to 72 hours.
Chloride-based environments are used for these corrosion tests as chloride is an aggressively corrosive ion and during the winter season it is normal practice to scatter sodium chloride on roads in order to facilitate the melting of ice and snow in order to make roads passable with a higher degree of safety. Thus the exposure of exterior automobile components to chloride ions can be very high.
In severe winter environments such as in northern Canada and Russia, sodium chloride is not sufficiently effective at snow melting and alternative salts have been used. Typical of these alternative salts are calcium chloride and magnesium chloride.
In the last few years, it has become apparent to the automotive industry that the use of calcium chloride represents a particular problem for chromium coatings. It is found that in environments where calcium chloride is used, salts can dry on the exterior of automobiles in combination with soils and mud. When this happens on a chromium coating, a particular type of accelerated corrosion occurs and the chromium deposit is effectively removed, leaving the nickel deposit exposed. This reduces the corrosion protection of the entire combination coating, and in addition, when the car is cleaned of these soils, the chromium deposit then looks unattractive as it exhibits dark spots, mottled appearance and yellow patches.
Thus automotive companies have a desire to improve the resistance of the chromium coatings to environments containing calcium chloride.